This invention relates to a method and apparatus for testing the compressibility of a sample, such as a subterranean core sample.
In the field of geological exploration for sources of petroleum reserves, natural gas formations, and the like, relatively accurate predictions of the type and quantity of such reserves may be obtained by analyzing samples derived from the subterranean formation. For example, if a core sample from the subterranean formation is obtained and analyzed for its compressibility and permeability, generally accurate indications may be made regarding the quantity of petroleum that may be present in a subterranean reservoir, the degree of difficulty involved in extracting the petroleum, the ability of the subterranean formation to support the various mechanical devices which are used to extract the petroleum, and the like.
Various techniques are known for testing or measuring the compressibility of core samples derived from the aforementioned subterranean formations. For example, the core sample may be placed in a testing chamber subjected to a relatively high pressure, and an axial load may be applied to the same by, for example, driving a mechanical plate downward onto the sample. The driving force may be generated by a pressure-activated piston, and sensing elements, such as strain gauges, have been used to determine the axial change in sample length at various loading conditions. Compressibility is determined as function of strain or length reduction, compared to the original sample length.
In another technique for measuring compressibility (and one in which the present invention finds ready application), a completely liquid-saturated core sample is placed in a chamber and a high confining pressure is applied to all surface areas of the sample, resulting in a condition of multiple-axis loading analogous to overburden loading in a natural reservoir. Another pressure source, in communication with the internal saturated pore-space of the sample only, is used to control the ratio of pore-space pressure to overburden sample-loading pressure, giving a direct measurement of net confining pressure.
In an untapped natural reservoir, the pore-space pressure and overburden loading pressure are in equilibrium: the combined hydraulic pressure of the pore-space fluid and the mechanical strength of the rock is equal to the overburden force imposed by the overhead rock structure. As fluids are withdrawn from the reservoir during production, there is a corresponding loss in hydraulic pore pressure, transferring a greater proportion of the overburden load to the rock structure. As the strength of a porous rock material depends in part on the area of contact between the individual grains within the matrix, a reduction in pore pressure will cause grain slippage to a point where the increased grain contact area provides a propping strength equal to the loss in hydraulic pore pressure. The resulting compressibility effect results in a corresponding reduction in pore volume as grains of material are driven into closer contact. Therefore, pore volume reduction and rock compressibility can be accurately determined by measuring the volume of fluid displaced from the saturated pore space, as a function of differential pressure.
Accurate measurement of the small volume of fluid displaced from the sample pore space may be achieved by withdrawing a small, uniform diameter rod inserted into a plumbing system connected to the pore space fluid chamber, to which a means for measuring pressure is included. A rod of known diameter withdrawn a known length provides a known volume in which pore fluid can be displaced. The resulting pore pressure is measured by the pressure reading devices, after grain deformation is complete and the sample has reached equilibrium. The resulting fluid displacement is compared with the total pore space volume of the sample for determination of compressibility at a known loading condition.
One disadvantage of compressibility testing devices of the aforenoted type, using a moving piston of small diameter and considerable length, essential to accurate volume determination, is the risk of having the piston buckle because of the compressional force exerted on it by the high fluid pressure applied to the pore space.